With the advent of digital video products and services, such as Digital Satellite Service (DSS) and storage and retrieval of video streams on the Internet and, in particular, the World Wide Web, digital video signals are becoming ever present and drawing more attention in the marketplace. Because of limitations in digital signal storage capacity and in network and broadcast bandwidth limitations, compression of digital video signals has become paramount to digital video storage and transmission. As a result, many standards for compression and encoding of digital video signals have been promulgated. For example, the International Telecommunication Union (ITU) has promulgated the H.261 and H.263 standards for digital video encoding. Additionally, the International Standards Organization (ISO) has promulgated the Motion Picture Experts Group (MPEG), MPEG-1, and MPEG-2 standards for digital video encoding.
These standards specify with particularity the form of encoded digital video signals and how such signals are to be decoded for presentation to a viewer. However, significant discretion is left as to how the digital video signals are to be transformed from a native, uncompressed format to the specified encoded format. As a result, many different digital video signal encoders currently exist and many approaches are used to encode digital video signals with varying degrees of compression achieved.
The primary objective of any digital video signal encoder is to achieve a high degree of compression without a significant loss of video signal. Video signal compression is generally achieved by representing identical or similar portions of an image as infrequently as possible to avoid redundancy. As a result, an image which has only very coarse detail and very few distinct colors can be compressed to a much smaller representation in comparison to a compressed representation of an image with significant amounts of very fine detail and many distinct colors. Unfortunately, video cameras and other video signal acquisition equipment introduce noise into the video signal and, from the perspective of video signal processing, the noise is generally indistinguishable from fine detail in the subject of the video signal. For example, ordinary noise in a monochromatic image may be indistinguishable from the fine detail and texture of a terrycloth towel photographed up close.
Digital video signal compression typically involves a transformation, e.g., a discrete cosine transformation (DCT), in which pixels which are relatively close in value to one another are represented in a particularly compact form. Noise in a digital video signal has a particularly adverse effect on such compression since the noise is frequently unrelated to the subject matter of the video image and frequently renders portions of the digital video signal inappropriate for representation in such a compact form.
To achieve both enhanced image quality and greater compression, video signal encoders frequently filter a video signal prior to encoding the video signal. However, the use of a particularly strong filter achieves greater compression at the expense of greater signal loss, and a particularly light filter preserves more of the original signal at the expense of a smaller degree of compression. Thus, digital video signals which include significant noise force a choice between image clarity and compression rate, i.e., the rate between the amount of data required to represent the digital video signal in uncompressed and compressed forms.
In addition, noise introduced by inexpensive, low-cost video capture and processing equipment is not adequately addressed by currently available digital video signal filters. Currently available digital video signal filters typically focus on Gaussian noise. However, ever growing popularity of inexpensive video recording and processing equipment, such as video cameras, video tape, and personal computer video capture cards, of moderate or questionable quality introduces noise which is not Gaussian.
What is needed is a digital video signal filter which can better eliminate the types of noise found in digital video signals without compromising the clarity and quality of the image of the digital video signal to thereby simultaneously improve the quality and compression rate of the digital video signal.